Various processes are known to remove contaminating gaseous components from a natural gas stream. The processes may be based on physical and/or chemical separation techniques. The physical separation techniques use differences in boiling, condensation and/or freezing points of the various contaminating components to selectively remove one or more of these components in a fractionating column, or differences in density to separate components with different densities in a centrifugal or cyclonic separator. The chemical techniques may employ selective absorption or catalytic reactions to convert a contaminating component into a composition that can be easily separated.
The standard technique for removing hydrogen sulphide and carbon dioxide from natural gas is amine treatment, which is based on solvent absorption. In this process the contaminating components are bound on a molecule such as diethanol amine in an aqueous solution. The clean hydrocarbon gas is not absorbed and emerges in the product gas stream. The solution with the absorbed contaminant is recycled and heated by approximately 100° C. to drive off the gases, which then are collected in a waste stream. The major cost factors in this process are the energy requirement for waste gas regeneration, solvent losses and the fact that the waste gases are regenerated at near atmospheric pressure—any process such as reinjection requires compression.
The operating costs for any gas purification process need to be a relatively small fraction of the value of the clean gas produced. Amine plants with their extensive gas-liquid contacting schemes will be fairly large, expensive and uneconomic if the gas stream contains a large fraction of contaminants.
Known gas separation centrifuges rotate at about 50,000 revolutions per minute (RPM) to separate gaseous fractions with only minor differences in density. These fast rotating centrifuges are known as ultracentrifuges and have limited separation efficiency and can only handle a limited flux of gas. If a large natural gas stream containing a large fraction of contaminants is to be purified by means of centrifuges then a large amount of centrifuges or ultracentrifuges are required, which renders centrifugal separation uneconomical.
U.S. Pat. Nos. 4,994,097; 5,221,300 and 5,902,224 and the paper ‘Phase separation in centrifugal fields with emphasis on the rotational particle separator’ published by J. J. H. Brouwers in the magazine Experimental Thermal and Fluid Science 26 (2002), pages 325-334 disclose centrifuges for separating solid impurities from gas mixtures.
It is an object of the present invention to provide a method for removing contaminating gaseous components, such as carbon dioxide and/or hydrogen sulphide, from a natural gas stream in an efficient and economical manner, even if the natural gas comprises a large fraction of contaminating gaseous components.